In order to improve living quality, humanity has invented numerous apparatuses such as automobiles, trains, airplanes, and other machineries. These machineries require fuels to operate and due to their vast abundance, fossil fuels such as petroleum oil and coal have been used. One unfortunate characteristic about fossil fuels is that their end products are pollutants, such as CO.sub.2 and carbon particulates, that have been found to affect living quality and the world's climate, known as the green house effect. With the rapid increase in the world's population, these natural resources have been found to deplete at a very rapid rate while their side effects have caused a great concern to humanity on both their health and living environment. This has motivated searches for renewable energy sources that can help to solve these problems. Oils from animals and plants have been found to ameliorate these problems to some extent and have been employed ever since. However, to meet the world demands for renewable energy, oils from animals and plants alone will be insufficient. For example, to meet the projected world's energy demands for biofuels by 2100 (351 ExaJoules/year), the entire agricultural land (about 1.5 billion hectares), will be needed (Huesmann, 2006). In the U.S. alone, it would take all 190 million-hectares of crop land (Kheshgi et al 2000) to produce ethanol from corn to replenish the gasoline need, which amounts to about 25% of the energy demand. This clearly indicates that other alternative sources for producing biofuels are in dire need. Due to their rapid growth (in hours rather than days or months) with minimal food and maintenance supports, algae, both macro and micro, appear to be the choice. During the energy crisis of the 1970s, the Aquatic Species Program (ASP) of the Department of Energy has identified 3000 species out of about 100,000 known algae species for development and they have finally narrowed them down to 300 species, both fresh and salt water varieties, for further works on oil producing capabilities.
Microalgae (or, for convenience, algae) are unicellular organisms that can grow very rapidly by photosynthesis using waste products and CO.sub.2. Besides using as a renewable biomass source for diesel fuel substitute (generally called biodiesel) and other biofuels such as methane, ethanol, and hydrogen, algae are also used as a source of proteins, fatty acids, and other biochemicals in food products as well as animal feed supplements. Further, they are used in the production of oxygen, vitamins, minerals, pharmaceuticals, and natural dyes. Gases released from fossil fuel production and utilization such as CO.sub.2, NO.sub.x, VO.sub.x can all be effectively scrubbed by algae, while nitrogen, phosphorous, and toxic substances in sewage treatment can also be removed by algae. Other applications of algae include biological control of agricultural pests and biodegradation of plastics. As a by-product, they are used as soil conditioners and biofertilizers in farming. This clearly indicates that a mass production of algae at low cost with high yield would be of great desire.
Two major advantages have been found in microalgae over other terrestrial plants such as soybeans, corn, jutropha, or palm. First, algae have a much higher growth rate and consequently their biomass productivities over space and time are significantly greater. Based on ash-free dry weight (AFDW, e.g., organic matter), the current algal yield is around 70 metric tons per hectare per year (mt/ha/yr), while the yields of soybeans, corn, and switchgrass or hybrid poplars are only 3, 9, 10-13 mt/ha/yr, respectively; accordingly, the footprint of biofuel systems in terms of both land and water is much smaller. Second, algae are more tolerant to varying environmental conditions. They can grow on non-arable land, using saline or brackish water. Many species of algae, such as Dunaliella, grow in seawater and can use CO.sub.2 from desulphurized flue gases of fossil-fuel fired power plants. Also, culturing algae in aqueous fluid instead of soil promotes their access to resources such as water, CO.sub.2, and minerals. For these reasons, about one-third of the net photosynthetic activity in the world is from algae. The combination of high biomass productivities, and the lack of needs for arable land and freshwater will enable the large-scale production of biofuels without disturbing food crops, and agricultural and forest land, while restoring the Earth from global warming problems.
Published oil yields of various species are: Chlorella minuttssima 31% (Illman et al., 2000); Isochrysis galbana 39% (Fidalgo et al., 1998); Nannochloropsis sp. 38% (Fábregas et al., 2004); and Tetraselmis suecica 30% (Otero and Fábregas, 1997). Some other algae such as Botryococcus bruinii have been found to yield as much as 75-80% of their dry weight.
The traditional practice in growing microalgae is to use a shallow raceway pond (Oswald 1960). This type of pond comprises of a curved-edge oblong rectangular pond with a middle island dividing the pond into two separate raceways. The pond is typically 20-30 cm deep and is driven by a large paddlewheel installed on one corner of the pond. The idea of being shallow is to allow maximum exposure of microalgae to sunlight. The idea of a paddlewheel is to keep microalgae agitated so it will be constantly exposed to sunlight. Unfortunately, neither of these ideas appears to be effective. With the pond being shallow, only small amount of microalgae can be grown in a fairly large area. With a large paddlewheel driving a large amount of fluid is neither economic nor effective. Most of microalgae would start to settle down toward the bottom of the pond as soon as it propelled away from the paddlewheel. From a calculation, most microalgae have an average of exposure time of only 3 minutes for an eight-hour of pond sun exposure. Attempts to improve yields and productivities of raceway ponds have been document in U.S. Pat. Nos. 6,192,833 on a partitioned aquaculture system and 7,198,940 on a controlled eutrophication process. These patents provide value-added applications to the raceway pond concept by augmenting them with a fish pond and a controlled eutrophication process with the basic weaknesses still remain.
Another pond design that has been considered is the deep-pond concept. This concept has not been generally accepted due to the fact that microalgae will most of the time (70%) remain in the dark region where sunlight cannot reach, which will consequently result in less growth.
To overcome both dilemmas discussed above, the concept of photobioreactor has been conceived and implemented. This concept attempts to provide sunlight exposure of microalgae by channeling microalgae fluid through a clear polyethylene tubing or pipe that is constantly exposed to sunlight through various configurations: vertical, sloped, horizontal, etc. (U.S. Pat. No. 5,162,051 and a review article by Yuan-Kim Lee on “Enclosed Bioreactors for the Mass Cultivation of Photosynthetic Microorganisms: The Future Trend”, TIBTECH, July 1986, pgs. 186-189). The concept, again, is also unfortunate, which often results in sunlight saturation, inhibition, and respiration, especially during high sunlight period of the day. High sunlight in a relatively short light path length (tube) can damage and/or kill algae cells. There have been several techniques in overcoming sunlight over exposure such as using cover shielding during peak hours, placing these tubular structures inside a green house, etc. Examples of recent attempts to improve the concept have been documented in U.S. patent application No. 60/932,674, filed May 31, 2007 and provisional application no. 20080311649. Again, these applications deal mainly with logistics and controls of the design in attempts to improve yields, but the basic concepts are still essentially the same. These techniques have worked to some extent, but either way, the concept of photobioreactors is basically rather costly and volume limited, which is often a major factor in large-scale commercial production. On large-scale production, photobioreactors will not only be too costly, but it is almost impossible to implement, even in a hybrid system that combines the concept of raceway pond with a photobioreactor. For example, on a very large raceway pond of, say, 4 acre-feet or pond of size 656′×164′×1.64′—176438 cuft or 1,376,215 gallons of microalgal fluid—it would take 95-143 days to circulate the microalgae fluid at 20-30 cm/sec using a 2″-diameter clear pipe. In order to achieve one complete cycle in a day, the flow rate of the fluid must be at least 13,759 gallons/minute for this 2″-diameter pipe, which will allow sunlight exposure of microalgae about only 0.1 minute/8 hour operation time. These numbers indicate impracticality of the system. The system will not only be rather expensive to implement, but its implementation may not work as well as it should be. The high flow rate may also cause damage to microalgal cells or it may deter proper growth of most microalgae.
The overall photosynthetic efficiency can be affected by many other factors for both existing raceway pond and photobioreactor approaches. These include the limited wavelength range of sunlight capable of driving photosynthesis (400-700 nm, which is being only 45% of the total solar energy), respiration requirements (during dark periods), efficiency of absorbing sunlight, and other growth conditions. So the need exists for a large-scale production system that can help to improve these efficiencies.